This invention generally relates to estimating in real time the instantaneous phase of a periodic signal. More specifically, this invention relates to an apparatus and method for mapping the relationship between (1) substantially periodic signals corresponding to the motion of a subject under examination in the course of an NMR scan and (2) a signal related to the phase of the motion, wherein the phase information is used in connection with a process for controlling image artifacts caused by the subject motion.
Subject motion during the acquisition of a nuclear magnetic resonance (NMR) image produces both blurring and artifacts. In the most commonly used two-dimensional Fourier transform (2DFT) or spin warp methods the artifacts are typically "ghosts" in the phase-encoded direction. Ghosts are particularly apparent when the motion is periodic, or nearly so. For most physiological motion, including cardiac and respiratory motion, each NMR spin echo or FID can be considered a snapshot view of the object. Blurring and ghosts are due to the inconsistent appearance of the object from view to view.
In projection reconstruction imaging techniques, substantially periodic motion also causes local distortion and blurring, as well as artifacts that extend well away from the moving structure. In these techniques, the artifacts are manifested as streaks rather than ghosts.
Both deleterious effects of periodic motion, blurring and artifacts, can be reduced if the data acquisition is synchronized with the periodic motion. This method is known as gated scanning. Gating can also be used to study the mechanical dynamics of the motion itself, if that is of interest. The drawback of gating is that, depending on the period of the motion, the fraction of the period during which acceptable data can be acquired, and the shortest acceptable pulse sequence repetition time, gating can significantly lengthen the data acquisition time.
While gating is required when the blurring due to the motion is unacceptable and when the motion itself is of interest (e.g., cardiac motion or flow), there are other applications where the loss of detail of the moving structures can be tolerated, but the disturbing effects of the artifacts which can extend far from the moving object cannot be accepted. In such applications, a method that can reduce or eliminate artifacts without the restrictions of gating is needed.
One method for reducing the undesirable effects due to periodic signal variations is disclosed and claimed in commonly assigned U.S. patent application Ser. No. 766, 842, filed Aug. 16, 1985 and U.S. Pat. No. 4,663,591, issued May 5, 1987. According to the method, the time sequence of views which collectively compose a scan is controlled in such a manner that when the views are reordered for construction of an image using Fourier transforms, the motion as seen in the "k-space" of the Fourier transforms is either very slow or very rapid. In the latter approach, the artifacts caused by respiratory motion are moved to the edge of the image where they may be moved out of the displayed field of view. In the former approach, the artifacts are minimally displaced from the moving object portions, thereby virtually eliminating most of the visually appreciable reduction in image quality. In implementing either approach as they are disclosed in the above-mentioned U.S. Pat. No. 4,663,591, substantially instantaneous knowledge of the phase of the respiratory motion is required in order to determine the best sequence of views or view order.
View order selection involves establishing the order in which either the variable amplitude phase-encoding gradient pulses (in the spin-warp method) or the direction of the read-out gradient (in multiple angle projection reconstruction method) are implemented. The view order is chosen so as to make the motion, which would appear to be at a particular frequency (as a function of the phase-encoding amplitude or gradient direction) if the normally used sequential view order was selected, appear to be at another frequency chosen so as to minimize the negative effects of the motion.
In the 2673 (Ser. No. 766,842) application, a view order for a scan is selected before the scan begins. Although this method is effective in reducing artifacts, and is in some respects ideal if the variation in the NMR signal caused by subject motion is rather regular and at a known frequency, the method is not very robust if the assumption made about the motion temporal period does not hold (e.g., because the patient's breathing pattern changes or is irregular). If this occurs, the method loses some of its effectiveness because the focusing of the ghost artifacts, either as close to the object or as far from the object as possible, is less complete.
In the 2916 application, improved results are achieved by abandoning a preselected order and, instead, constructing an order of views in response to the measured motion occurring as the scan is executed. To achieve artifact reduction, a desired relationship between motion phase and phase encoding amplitude is selected. As the scan data are acquired, signals representative of the position of the object are measured and used to select a view order that satisfies the desired relationship between motion phase and phase encoding amplitude. The detailed map between object position and phase encoding amplitude will depend on the fraction of the time that the object spends at each particular position.
For example, the end expiration portion of the respiration cycle is of different relative lengths in different people, and even at different times in the same person. Clearly since data are to be acquired at a regular rate, a larger fraction of the views must be assigned to end expiration in those cases where the end expiration portion of the cycle is longer. In the method of the 2673 application, this is trivial since perfect knowledge of the motion is assumed. In many cases, however, such knowledge cannot be assumed. Also, the motion pattern might vary during the examination.
In order to remove the burden of compensating for the details of the motion pattern, it is desirable to have a method and apparatus that converts the measured motion signal into a new signal, herein referred to as "phase", which the view order selector can use readily. Because the view order selector need no longer worry about the details of the motion pattern, it can very quickly and accurately select the view order for the scan as the scan progresses. Since the respiratory pattern of some subjects is known to vary during the examination, it is also desirable to have a method than can adapt or "learn" as the motion pattern varies. As will be shown, the analog signal generated by conventional respiratory monitors cannot directly be used to quickly and efficiently select the view order.